Add a postinstall section to leave a trace in the home directory.

This PR adds the post-installation section that adds a file
in the /home space, which then can be checked by a `preserve_home`
post-installation test and prove that it was possible to
preserve home on btrfs layouts.

desktop.ks

@@ -20,3 +20,7 @@ poweroff
 @^workstation-product-environment
 -selinux-policy-minimum
 %end
+
+%post
+touch $INSTALL_ROOT/home/home_preserved
+%end

minimal-uefi.ks

@@ -14,3 +14,7 @@ poweroff
 %packages
 @core
 %end
+
+%post
+touch $INSTALL_ROOT/home/home_preserved
+%end